Study: Heat Improves Cool Roof Effect

A recent study suggests that white roofs can decrease the amount of urban heat island effect during a heat wave, according to research from the Lawrence Berkeley National Laboratory.

The study, which examined how white roofs reflected the sun in Guangzhou, China, found that urban midday temperature was lowered 1.2 degrees Celsius instead of 0.8 degrees Celsius—or 50 percent—compared to the reduction for typical summer conditions.

Lab researchers used a regional climate model combined with an urban model that let them adjust roof reflectance to achieve the results, according to a recent statement from the lab.

Photos: Berkeley Lab

Lab researchers used a regional climate model combined with an urban model that let them adjust roof reflectance to achieve the results, according to a recent statement from the lab.

“The hotter it is, the more cooling you get with cool roofs—and it is a significant difference, compared to the margin of error,” said Dev Millstein, one of the authors of the study. “We found that the stagnant conditions of a heat wave, where the air is just sitting over the city, was one of the main factors.

“Looking at the average difference in temperature between every grid cell in the city and the adjacent rural area, cool roofs had a more dramatic effect during heat waves,” Millstein said.

Reflective roofs, also called cool roofs, save energy by keeping buildings cooler, according to the lab. The result is a more efficient design that relies less on air conditioning.

Energy and Health Benefits

Hot surfaces, such as dark roofs, warm the outside air and add to the urban heat island effect the lab said. Its previous research in China found that cool roofs could reduce energy use and greenhouse gas emissions in climate zones with hot summers.

That means studying heat waves not only contributes to energy efficiency but also to improved health, the lab said.

Hot surfaces, such as dark roofs, warm the outside air and add to the urban heat island effect, the lab said. White roofs, on the other hand, help mitigate this effect because they reflect the sun.

“That’s when reducing the hottest temperatures can have the most health benefit,” Millstein said. “It’s also when the electric grid is the most stressed. Air conditioners are running at full speed and with no break, so a small change on the margin can have a bigger impact.”

Research Data

Guangzhou, in southern China, has a population greater than 8.5 million. Researchers simulated conditions from six of the strongest heat waves over the past decade and compared them to 25 typical summer weeks between 2004 and 2008.

The team made the roofs in the study have a reflective similarity to an aged white roof. Although the lab noted that it is unlikely all roofs in the city will be aged white roofs, designing the experiment in this manner was necessary to have a statistically significant signal.

The research was funded by DOE’s Building Technologies Office, through the U.S.-China Clean Energy Research Center Building Energy Efficiency (CERC-BEE), the Chinese Academy of Sciences, and the National Natural Science Foundation of China.

In addition to Millstein, the lab researchers on the study were Ronnen Gevinson and Pablo Rosado. Meichun Cao and Zhaohui Lin, of the Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, also worked on the project.